Three piston rings, a top ring, a second ring, and an oil ring that are used in an internal combustion engine, are disposed in such a manner that each of them engages with a ring groove provided on a surface of a piston and serve a gas sealing function of preventing a combustion gas leak from a combustion chamber, a heat conduction function of transmitting heat of the piston to a cylinder wall which is cooled down and thereby cooling the piston down, and a function of scooping out excess oil by applying an appropriate amount of engine oil serving as lubrication oil to the cylinder wall, respectively.
These three piston rings, during operation of the internal combustion engine when the piston is reciprocated by explosion of fuel in the combustion chamber, repeat colliding against a surface of the ring groove inside the ring groove of the piston. Also, since the piston ring is slidable in a circumferential direction thereof inside the ring groove, the piston ring slides inside the ring groove. However, on the surface of the ring groove, a projection of approximately 1 μm in height is formed by machine turning in formation of the ring groove. As the projection slides colliding against the piston ring, the projection becomes worn away, exposing an aluminum surface in the surface of the ring groove.
When the exposed aluminum surface further repeats sliding and colliding against a lateral surface of the piston ring, aluminum cohesion, a phenomenon in which aluminum alloy attaches to the lateral surface of the piston ring, occurs. This phenomenon is especially noticeable at the top ring located proximate to the combustion chamber and exposed to high temperature.
When the aluminum cohesion further progresses, the wearing of the ring groove rapidly progresses, enlarging a gap between the piston ring and the ring groove. As a result, the gas sealing function of the piston ring is degraded, causing a phenomenon what is called blowby in which high pressure combustion gas flows out to a crank chamber from the combustion chamber and leads to a reduction in engine power.
As such, there have been suggested a variety of techniques for preventing the aluminum cohesion of the piston ring. For example, PTL 1 describes a technique to provide a resin-based film containing carbon black particles to an upper side face and a lower side face of the piston ring that collide against and slide on the ring groove, thereby improving conformability and preventing the aluminum cohesion.
Also, PTL 2 describes a technique to provide a heat-resistant resin containing, relative to an entire coating film on the surface, 10 to 80 mass % of one or more powder selected from a group constituted by nickel-based powder, lead-based powder, zinc-based powder, tin-based powder, and silicon-based powder to at least one of the upper side face and the lower side face of the piston ring, thereby effectively preventing the aluminum cohesion to the piston ring.
However, the films described in PTLs 1 and 2 have a problem that, when temperature inside an engine rises, aluminum cohesion resistance is degraded. As such, PTL 3 describes a technique to provide a polyimide film containing hard particles and having a solid lubricant function to at least one of the upper side face and the lower side face of the piston ring, thereby maintaining high aluminum cohesion resistance for a long time under a condition with high temperature over 230° C.
Further, PTL 4 describes a technique to provide, in place of the resin-based film, a first diamond-like-carbon (DLC) coating film containing at least silicon and a second DLC coating film containing at least W, or W and Ni formed under the first DLC film, to the upper side face and the lower side face of the piston ring, thereby providing a piston ring having excellent aluminum cohesion resistance, scuff resistance, and abrasion resistance.